In a nonaqueous electrolyte for a secondary cell, a lithium salt is usually well dissolved to provide a high lithium ion conductivity, and a carbonate type compound has been widely used as a solvent from such a viewpoint that it has a wide potential window. However, a carbonate type compound usually has a low flash point and is easily flammable at the time of e.g. runaway of a cell, and therefore, it has a serious problem from the aspect of safety. To overcome such a problem, many nonaqueous electrolytes have been proposed in which a lithium salt having a chain structure such as LiPF6 or Li[N(SO2C2F5)2], a carbonate type solvent and a hydrofluoroether having no flash point, are used in combination.
As such a hydrofluoroether, a monoether of an alkyl group and a fluoroalkyl group (a fluoroalkyl group is a group having at least one hydrogen atom in an alkyl group substituted by a fluorine atom), or a specific polyfluoroether having a chain structure and having at least two etheric oxygen atoms, is known.
A nonaqueous electrolyte containing such a hydrofluoroether is, in its initial composition, not ignited even when contacted with an ignition source. The reason is such that a low boiling point hydrofluoroether having no flash point is filled in the vapor phase to provide so-called choking effect. However, with a conventional nonaqueous electrolyte containing a hydrofluoroether, if an excessive load is exerted to the cell, and a high temperature condition is prolonged, such a hydrofluoroether volatilizes to lose the choking effect, whereby it has been sometimes difficult to warrant nonflammability.
Therefore, the following nonaqueous electrolytes have been proposed as nonaqueous electrolytes to warrant nonflammability more securely.
A nonaqueous electrolyte for a lithium secondary cell comprising LiPF6 and an organic fluorinated ether compound represented by R7—O—R8 (wherein R7 and R8 are fluorinated alkyl groups) (Patent Document 1).
A nonaqueous electrolyte comprising LiN(CF3SO2)2 and, as a solvent, a glyme type solvent (Patent Document 2):
A nonaqueous electrolyte comprising LiBF4 and, as a solvent, tetraglyme (Patent Document 3),
On the other hand, it has been reported that lithium salts such as CF3SO2N(Li)SO2CF3 and FSO2N(Li)SO2F form a strong interaction with etheric oxygen of the glyme type solvent to form a stable 1:1 complex, and from the results of e.g. the thermal analysis, such a complex exhibits a behavior as if it were a single ion species and was not ignitable at all by heating by a burner (Non-Patent Documents 1 and 2).
Further, a nonaqueous electrolyte is disclosed which comprises a glyme type solvent, an alkyl(fluoroalkyl)ether represented by CaFbOR9 (wherein R9 is an alkyl group, a is an integer of from 1 to 10, and b is 2a+1), a cyclic carbonate and a lithium salt represented by LiN(SO2(CF2)cF)2 (wherein c is an integer of from 1 to 5) (Patent Document 4).